1. Field of the Invention
The present invention relates to improvements in decoking residual oil delayed cokers and to apparatus decoking.
2. Description of the Prior Art
It is well known that during delayed coking, a stream of residual oil passes through the pipes of a heater at a high flow rate, where the residual oil is heated to the temperature required by the coking reaction, and then enters a coker where it undergoes reactions such as cracking, condensation, etc. with the help of its entrained heat. The oil-vapors produced thereby are introduced into a fractionating column for fractionation and the coke deposited in the coker is periodically removed after it has accumulated to a given height (Hydrocarbon Processing, Vol. 50, No. 7, 1971).
Early decoking of the delayed coker was carried out using a coiled steel rope, which was inserted into the empty coker from top to bottom and then drawn out with a hoist after the coke accumulated in the coker to a given height. This decoking procedure was time-consuming, ineffective, and labor-intensive.
In order to overcome the above-mentioned disadvantages, a hydraulic decoking technique using a derrick was proposed. This process, invented in the United States during the 1930's, is still widely used for decoking in the delayed coking process.
The afore-mentioned hydraulic decoking technique employs a high speed, high impact water jet to remove coke from a coker, which process essentially consists of two operations: bore drilling and coke cutting. In this hydraulic decoking process, a derrick, measuring about 40 m in height and positioned on the top platform of the coker, is used deliver high pressure water through a hollow drill rod supported on the derrick to a coke remover. A high pressure water pump forces water through a high pressure rubber hose. Using drilling means, a hole is bored in the coke accumulation and finally high pressure water is ejected from the nozzle of the coke remover to accomplish decoking (Petroleum Processing, Vo. 5, No. 2, 1950).
In addition to the above-noted components, operation of this hydraulic decoking technique requires a hoist, overhead crane, sling hook, and other associated means. The drill rod and coke remover are brought into rotation by an air-operated motor. The derrick and hoist are used to raise and lower the drilling means and the coke remover, as desired, with the help of the steel ropes (Petroleum Processing, Vo. 5, No. 2, 1950).
Compared to the early steel rope decoking process, the hydraulic decoking technique is more efficient, safer, and environmentally cleaner; environment. However, it should be noted that this hydraulic decoking technique requires the use of a considerable amount of heavy, structurally complicated equipment, steel, and major expenditures, since each coker must be equipped with a steel derrick and its own coke removing apparatus. Additionally, the 40 m height of the derrick hinders operation and maintenance.
Recently, the development of the delayed coking process has resulted in cokers with larger diameters and harder, high quality coke; therefore, it is necessary to correspondingly raise the pressure and flow rate of the high pressure water jets used for removing coke. Accordingly, continuous improvements have been made on the decoking process and apparatus as embodied, for example, in U.S. Pat. Nos. 3,412,012 and 3,836,434.
U.S. Pat. No. 3,412,012 discloses a decoking process wherein a high, above-ground derrick is needed. In addition, the drill stem must be kept continuously rotating. When the coke accumulates to a given height, the coke remover performs decoking by ejecting a high pressure water jet. However, this procedure increases energy consumption, a significant disadvantage of this decoking process.
According to U.S. Pat. No. 3,836,434, a central bore is drilled and then high pressure water is ejected against the coke accumulation from top to bottom in order to decoke by "peeling" or enlarging the central bore diameter. This apparatus, which includes a conducting mechanism, control means and valve, is complex and the operator cannot automatically switch between drilling and cutting operations, thereby requiring frequent adjustments and lengthening the procedure. Furthermore, a derrick is also required.
Both the above-described and subsequent hydraulic decoking techniques utilize a rigid drilling means, the process and apparatus thereof have numerous disadvantages including:
1. More structurally complicated equipment and high investment costs associated with using a derrick.
2. Switching between bore drilling and the coke cutting is not automatic, complicating operation and limiting the efficiency of decoking.